In the construction industry, it is not only well-known, but it is preferred that concrete structures and/or components be precast at the factory, and delivered to a job site for installation. Precasting has many advantages over in situ casting, the primary factors of which include the casting of components having greater precision and tighter tolerances, the consistency of product's specifications and reduced costs, which go hand-in-hand with use of mass assembly techniques as compared with one-at-a-time in situ casting.
Nevertheless, precast concrete structures of sizable dimensions are quite heavy, in many cases weighing thousands of pounds. For example, conventional precast manhole sections typically range in weight from 800 to 16,400 pounds each, necessitating the use of mechanical lifting and moving equipment.
Apparatus employed in one preferred technique for handling precast manhole sections is described in U.S. Pat. No. 4,088,361, which is incorporated herein by reference thereto. As is described therein, precast manhole sections are each provided with a pair of plastic inserts, which are releasably mounted upon an outer wall of a mold assembly in which a manhole section is cast so as to be embedded into the cast member.
The plastic insert has a cavity of a predetermined shape configured to receive a lift pin. At least two such insert assemblies are provided in each cast member, being arranged diametrically opposite one another. Additional inserts may be used, when needed, to handle cast members of greater weight, the inserts being arranged at spaced intervals about the cast member.
A lift pin in inserted into each insert. A lifting apparatus for lifting and moving the precast section, applies lifting tension to the lift pins, which cause the pins to rotate 90.degree. from an initial horizontal ("9 o'clock") position to a vertical (12 o'clock") position where they become locked within the inserts and hence the cast member to assure that the pins are not dislodged during lifting and movement of the precast member to assure that these operations are performed in a safe manner.
When the precast member is delivered to the desired location, for example, upon the flat bed of a truck for delivery from factory to installation site, the lifting mechanism moves the precast member to an installation site and is lowered, which relaxes the lifting tension at which time the lift pins are rotated from the vertical to the horizontal position for easy removal.
Although concrete has a maximum compressive strength of 4,000 PSI, as the diameter and wall thickness of manhole sections increases, the cantilever forces developed by the lift pin and exerted upon the lift pin insert and hence the cast member increase in magnitude due to the increased weight of the cast member which causes the cast material to fracture.
One technique which has been employed by the Applicant for increasing the structural supporting strength of the insert is to wrap a rebar about the periphery of the plastic insert so that the rebar forms a substantially V-shaped configuration with a loop being provided about the plastic insert at the apex of the V-shaped rebar. This arrangement has provided only a small improvement in the amount of load which a cast member can withstand before fracturing.
In addition, it is extremely advantageous to provide lift pin and insert assemblies which are usable in cast members having a wide range of weights and sizes, thereby reducing if not substantially eliminating, the need to provide a large inventory of lift pins of different sizes to accommodate the lifting and handling of cast members of different sizes and weights.